He could be referring to some of Gandalf's recollections from "The Council of Elrond." (still one f my favorite LOTR chapters.)

Perhaps told as this chapter is in the book, more from an Elvish/Elrond POV. (The film is from Frodo's POV.) We could see flashbacks to Elrond's memory of marching out with the Last Alliance host.

Also, if you are tying events in neatly to LOTR, a "natural progression", than many of Gandalf's recalled events leading up to the War would make perfect sense onscreen....the capture of Gollum, Aragorn leading him to Lorien, and the Elves' leniency with him that leads to his ecape. Just as PJ showed us His escape from Orthanc, etc....GDT could build on that somehow. (OMG....OMG...he may call Viggo back, and we'll see Aragorn and Gollum onscreen together.....*THUNK* . This would be pperfectly permissble from the TE's POV, he is of course Aragorn and not Thorongil at this point....so no invention.....(HAVE To go back and re-read "The Council of Elrond"...OMG....I'm going to faint.....(Might be interesting to see Viggo go "Disney" again after baring all....hehe. OH no....I am getting sinful thoughts...must stop)